In wireless communication systems, including New Radio (NR, sometimes referred to as 5G) and likely beyond, user equipment computes channel estimates based on pilot or reference signals. For example, downlink reference signals are predefined signals occupying specific resource elements within the downlink time-frequency grid. There are different types of downlink reference signals that are transmitted in different ways and used for different purposes by the receiving terminal, including channel-state information reference signals (CSI-RS) and demodulation reference signals (DM-RS, or DMRS, without the hyphen).
In NR, the orthogonal frequency-division multiplexing (OFDM) waveform is used for both downlink and uplink transmissions. The transmit signals in an OFDM system can have high peak values in the time domain, because the many subcarrier components are added via an iFFT (inverse Fast Fourier Transform) operation. As a result, OFDM systems are known to have a high PAPR (Peak-to-Average Power Ratio), compared with single-carrier systems.
Indeed, the high PAPR is one of the most detrimental aspects in an OFDM system, as a high PAPR decreases the SQNR (Signal-to-Quantization Noise Ratio) of ADC (Analog-to-Digital Converter) and DAC (Digital-to Analog Converter) while degrading the efficiency of the power amplifier in the transmitter. For example, an increase in PAPR can result in operating a device in the saturation region of the device's power amplifier. This results in a number of issues, including that such a device cannot be operated according to specified limits.
One solution to mitigate high PAPR is use power back off, in which the power amplifier needs to be backed off by some number of decibels dBs for NR operation. However, with power back off, the coverage is reduced, while at the same time the power amplifier efficiency is reduced. Another solution to reduce the PAPR is to use clipping, where the peaks are limited. However, with clipping the transmitted Error Vector Magnitude (a measure of signal quality) is increased, and the transmitted signal is distorted, which results in reduced throughput, particularly for higher order modulation schemes such as 64QAM, and 256 QAM.